Effects of vaccination against systemic Serratia infection

Host defense against Serratia marcescens in experimental infection in mice was enhanced by vaccination with formalin-killed bacteria of the same strain. The enhancement appeared within 24 hr after vaccination, reached a peak seven days later and lasted four weeks. The enhanced resistance to Serratia infection was also observed in the early phase (within seven days) after vaccination with killed Escherichia coli or other Gram-negative strains, but the efficacy on Day 7 was inferior to that with killed S. marcescens. Phagocytic activities of both circulating neutrophils and peritoneal macrophages were measured by the chemiluminescence (CL) response, and the activity of tissue macrophages was evaluated by the carbon clearance test. The activities were significantly elevated in the early phase, that is, within two or three days for neutrophils, seven days for peritoneal macrophages and at least 14 days for tissue macrophages, after vaccination with killed Gram-negative bacteria. These results suggest that the enhancement of host defense in the early phase is dependent on phagocytic functions that are non-specifically activated by dead bacteria. In the late phase after vaccination, specific immunity might have been involved in the defense mechanism. However, transfer of high titer specific antiserum, in itself, did not render mice resistant to Serratia infection.